It comes down to the contractor.The guy putting in the water probly has every thing on hard pipe so theres no give.Then the heads are probly orbit so they stick.Then the guy on the mower does zero turns instead of 3 points.He probly lets the front tires just whip around as well.He hits the heads with the edger.Small mowers less stress less production.Take your pick.I like my z but in cases where I see to much stress I use my walk behind.I would fire the guy for not telling me he hit the heads not for hitting them.Then get a good irragation man to fix the system.Not repair it but fix it.